This invention relates to integrated circuits (ICs) in general, and more particularly to trench capacitors. Such ICs include, for example, memory ICs such as random access memories (RAMs), dynamic RAMS (DRAMs), synchronous DRAMs (SDRAMs), static RAMs (SRAMs), and read only memories (ROMs) or other memory ICs. Other ICs include devices such as programmable logic arrays (PLAs), applications specific ICs (ASICs), merged logic/memory ICs (embedded DRAMs, or eDRAMs), or any circuit devices utilizing trench capacitors. Specifically, this invention relates to a new method for forming a buried plate in a trench capacitor, particularly a deep trench capacitor.
Integrated circuit (ICs) or chips employ capacitors for charge storage purposes. An example of an IC that employs capacitors for storing charge is a memory IC, such as a DRAM chip. Typically, a DRAM memory cell comprises a transistor connected to a capacitor. One type of capacitor that is commonly employed in DRAMs is the trench capacitor. A trench capacitor is a three-dimensional structure formed in the substrate, and typically comprises a deep trench etched into the substrate. The trench is filled, for example, with n-type doped poly. The doped poly serves as one electrode of the capacitor (referred to as the “storage node”). An n-type doped region surrounds the lower portion of the trench, serving as a second electrode. The doped region is referred to as a “buried plate.” A node dielectric separates the buried plate and the storage node.
Formation of the buried plate is an important part of the process for trench DRAM technology to improve the device performance. A conventional technique for forming the buried plate includes outdiffusing dopants into the region of the substrate surrounding the power portion of the trench. The dopant source is typically provided by an n-type silicate glass such as, for example, arsenic doped silicate glass (ASG). The buried plate is conventionally formed by deposition of a thin layer of ASG on the sidewalls of the lower trench followed by thermal anneal, known as the “drive-in” process. The ASG layer acts as an arsenic source to dope the buried plate.
As the feature size of trench technology decreases, especially as the trench size shrinks, the thickness of the ASG layer must be reduced accordingly. However, excessive reduction of ASG thickness causes lower arsenic concentration in the buried plate region because of the depletion of arsenic in the thin ASG layer. Lower arsenic concentration degrades device performance. Therefore, scaling of trench technology is severely constrained by the conventional ASG process.
A collar is also imperative for both buried plate and capacitance enhancement because it acts as a hardmask to block arsenic diffusion and silicon etching on the trench top portion. Conventionally, the collar is formed before deposition of the ASG. Several collar schemes have been developed, including the anti-collar scheme, the sacrificial poly scheme and the modified anti-collar scheme, but each of them has its inherent limitations.
In the anti-collar scheme, an oxide layer is formed first on the trench sidewall. Then the trench is filled with resist, and the top surface of the resist is recessed to a predetermined depth below the top of the trench. The oxide is removed from the exposed sidewalls in the top portion of the trench, then the resist in the lower portion of the trench is stripped. This leaves only the bottom portion of the trench sidewalls covered by oxide. Next, the wafer is exposed to a nitrogen-containing atmosphere, such as NH3. A thin layer of nitride is thermally grown only on the top portion of the trench sidewalls, because the bottom portion is covered by the oxide. Lastly, the oxide is removed from the bottom portion of sidewalls, leaving a nitride collar on the top portion only.
An inherent limitation of the anti-collar scheme is that the maximum nitride thickness by thermal growth is only about 25 Ã□ or less, which is not sufficient to act as a collar for subsequent processing.
The sacrificial poly scheme begins with formation of a first oxide layer on the trench sidewall typically by thermal oxidation. A first nitride layer is then formed on the trench sidewall by low pressure chemical vapor deposition (LPCVD). The trench is then filled with polysilicon, and the top surface of the poly is recessed to a predetermined depth below the top of the trench. Using in-situ steam growth (ISSG), the poly and nitride surfaces are oxidized, and then a second layer of nitride is deposited by LPCVD. Anisotropic etch by reactive ion etching (RIE) is then used to remove the nitride and ISSG oxide on the poly, while leaving the nitride and ISSG oxide on the trench sidewall. An aggressive etch follows in order to remove all of the poly in the trench bottom portion. The first nitride layer on the trench bottom is then stripped, stopping on the first oxide layer. Simultaneously, the second nitride layer on the trench top portion is stripped. The first oxide layer on the trench bottom portion and ISSG oxide on the trench top portion are then stripped, leaving a collar formed on the trench top portion only. The collar includes a thin layer of oxide and a layer of nitride.
Unlike the anti-collar scheme in which the nitride collar is formed by thermal growth, the nitride in the sacrificial poly scheme is formed by LPCVD. Therefore, the nitride in the sacrificial poly scheme may be any thickness. This scheme, however, suffers from a couple of disadvantages: process complexity, and severe defect generation during poly removal from the trench bottom. The removal process is a very aggressive etch process in order to completely remove the polysilicon. This may cause severe defect issues such as pinholes on the trench sidewall and damage on some areas such as alignment marks.
In the modified anti-collar scheme, a first oxide layer is formed on the trench sidewalls by thermal growth. Then, a first nitride layer is formed on the trench sidewall by LPCVD. A second oxide layer is next formed on the trench sidewall by LPCVD. A thin layer of polysilicon is deposited on the trench sidewall by LPCVD, and then its surface is oxidized to form a third oxide layer. The trench is then filled with resist, and the top surface of the resist is recessed to a pre-determined depth below the top of the trench. The third oxide layer is removed from the exposed top portion of the trench sidewall, then the resist is stripped. A second nitride layer is formed on the trench top portion only by thermal nitridation. The bottom portion of the trench is covered by the third oxide, thereby inhibiting nitride growth on the trench bottom portion. The third oxide is then stripped from the trench bottom portion, using a removal process which is selective to the second nitride layer. The poly is then stripped from the trench bottom portion, using a removal process which is selective to the second nitride layer. The first nitride layer is then stripped from the trench bottom portion, stopping on the first oxide layer. Simultaneously, the second nitride layer on the trench top portion is stripped, stopping on the poly layer. The poly layer is then stripped from the trench top portion. The first oxide layer is then stripped from the trench bottom portion. Simultaneously, the second oxide layer on the trench top is stripped. A collar is thereby formed on the trench top portion only. Similar to the sacrificial poly scheme, the collar includes a thin layer of oxide and a layer of nitride.
The advantage of the modified anti-collar scheme is that it avoids the aggressive poly removal step. However, disadvantages include: process complexity, and poor quality of the collar with “pinholes” because of the quality of the thin films. The multiple layers of film deposition may cause a high defect density in the nitride collar. This scheme also suffers possible pinch-off in narrow trenches.
Moreover, the bottle shape of the trench is formed before the buried plate in all of these schemes. The collar is possibly broken during the bottle process, leading to its ineffectiveness for the subsequent buried plate formation process.
For example, the collar may be formed on the upper sidewalls of the trench using a sacrificial material in the lower region of the trench. In U.S. Pat. No. 6,509,599, polysilicon 152 is deposited over the wafer in order to fill the trench 108 (col. 6, lines 16–17). The polysilicon 152 is then removed down to the bottom side of the collar to be formed (col. 6, lines 27–28). A dielectric layer is then deposited over the wafer, covering the trench sidewalls (col. 6, lines 41–42). The dielectric layer is etched in order to form the collar 168 (col. 7, lines 1–3). Then, the polysilicon sacrificial layer 152 is removed (col. 7, lines 12–13). After the polysilicon has been removed, the buried plate is formed (col. 7, lines 58–60). A similar process utilizing polysilicon as the sacrificial material is described in U.S. Pat. No. 6,319,788.
Alternatively, an oxide material may be used as the sacrificial material. In U.S. Pat. No. 6,297,088, an oxide layer 112 is formed on the substrate 102 and into the trench structure 110, then an etching back step is performed to remove the oxide 112 above the top surface of substrate 102 and a portion of oxide 112 from the trench, thereby exposing upper sidewalls 111a (col. 4, line 63 col. 5, line 7). Collar nitride spacers 116 are next formed on the upper sidewalls 111a (col. line 22–24). Then, oxide 112 is removed (col. 5, lines 37–40), and doped areas 117 are formed in the bottom 110b and the lower sidewalls 111b of the trench structure (col. 5, lines 48–49). A similar process utilizing a sacrificial dielectric material is described in U.S. Pat. No. 6,365,485.
In the methods described above, the deep trench is fully formed in the substrate, and then a sacrificial material is deposited into the trench and recessed to expose an upper portion of the trench sidewalls. The collar is formed on the upper sidewalls, the sacrificial material is removed, and the buried plate is formed in the lower portion of the trench. In an alternative method, the deep trench is partially formed, the collar is formed on the sidewalls of the partial trench, the complete trench is then etched, and the buried plate is formed in the trench sidewalls below the collar. For example, in U.S. Pat. No. 6,225,158, upper portion 39 of the deep trench is lined with nitride collar 43 (col. 3, lines 3–6), then etching of the deep trench is completed (col. 3, lines 10–13). The deep trench is lined with ASG 49, and drive-in forms an n+ diffusion plate 51 surrounding the lower portion 47 of the deep trench (col. 3, lines 14–20). A similar process is described in U.S. Pat. No. 6,190,988.
Again, in all of these schemes, the bottle is formed before the buried plate. The collar is possibly broken during the bottle process, leading to its ineffectiveness for the subsequent buried plate formation process. Moreover, the buried plate is formed by deposition of a thin layer of ASG on the sidewalls of the lower trench followed by thermal anneal, known as the “drive-in” process. As the feature size of trench technology decreases, especially as the trench size shrinks, excessive reduction of ASG thickness causes lower arsenic concentration in the buried plate region because of the depletion of arsenic in the thin ASG layer, thereby degrading device performance. Therefore, scaling of trench technology is severely constrained by the conventional ASG process.
Therefore, there remains a need in the art for a method of forming a buried plate in a trench capacitor which does not rely on deposition of a thin layer of a dopant source film on the sidewalls of the trench, and which also does not rely on formation of the collar structure prior to deposition of the dopant source material.